Anders Warén

Biodiversity on the Rocks: Macrofauna Inhabiting Authigenic Carbonate at Costa Rica Methane Seeps

Carbonate communities: The activity of anaerobic methane oxidizing microbes facilitates precipitation of vast quantities of authigenic carbonate at methane seeps. Here we demonstrate the significant role of carbonate rocks in promoting diversity by providing unique habitat and food resources for macrofaunal assemblages at seeps on the Costa Rica margin (400–1850 m). The attendant fauna is surprisingly similar to that in rocky intertidal shores, with numerous grazing gastropods (limpets and snails) as dominant taxa. However, the community feeds upon seep-associated microbes. Macrofaunal density, composition, and diversity on carbonates vary as a function of seepage activity, biogenic habitat and location. The macrofaunal community of carbonates at non-seeping (inactive) sites is strongly related to the hydrography (depth, temperature, O2) of overlying water, whereas the fauna at sites of active seepage is not. Densities are highest on active rocks from tubeworm bushes and mussel beds, particularly at the Mound 12 location (1000 m). Species diversity is higher on rocks exposed to active seepage, with multiple species of gastropods and polychaetes dominant, while crustaceans, cnidarians, and ophiuroids were better represented on rocks at inactive sites. Macro-infauna (larger than 0.3 mm) from tube cores taken in nearby seep sediments at comparable depths exhibited densities similar to those on carbonate rocks, but had lower diversity and different taxonomic composition. Seep sediments had higher densities of ampharetid, dorvilleid, hesionid, cirratulid and lacydoniid polychaetes, whereas carbonates had more gastropods, as well as syllid, chrysopetalid and polynoid polychaetes. Stable isotope signatures and metrics: The stable isotope signatures of carbonates were heterogeneous, as were the food sources and nutrition used by the animals. Carbonate δ13Cinorg values (mean = -26.98‰) ranged from -53.3‰ to +10.0‰, and were significantly heavier than carbonate δ13Corg (mean = -33.83‰), which ranged from -74.4‰ to -20.6‰. Invertebrates on carbonates had average δ13C (per rock) = -31.0‰ (range -18.5‰ to -46.5‰) and δ15N = 5.7‰ (range -4.5‰ to +13.4‰). Average δ13C values did not differ between active and inactive sites; carbonate fauna from both settings depend on chemosynthesis-based nutrition. Community metrics reflecting trophic diversity (SEAc, total Hull Area, ranges of δ13C and δ15N) and species packing (mean distance to centroid, nearest neighbor distance) also did not vary as a function of seepage activity or site. However, distinct isotopic signatures were observed among related, co-occurring species of gastropods and polychaetes, reflecting intense microbial resource partitioning. Overall, the substrate and nutritional heterogeneity introduced by authigenic seep carbonates act to promote diverse, uniquely adapted assemblages, even after seepage ceases. The macrofauna in these ecosystems remain largely overlooked in most surveys, but are major contributors to biodiversity of chemosynthetic ecosystems and the deep sea in general.

A Source-Sink Hypothesis for Abyssal Biodiversity

Bathymetric gradients of biodiversity in the deep‐sea benthos constitute a major class of large‐scale biogeographic phenomena. They are typically portrayed and interpreted as variation in α diversity (the number of species recovered in individual samples) along depth transects. Here, we examine the depth ranges of deep‐sea gastropods and bivalves in the eastern and western North Atlantic. This approach shows that the abyssal molluscan fauna largely represents deeper range extensions for a subset of bathyal species. Most abyssal species have larval dispersal, and adults live at densities that appear to be too low for successful reproduction. These patterns suggest a new explanation for abyssal biodiversity. For many species, bathyal and abyssal populations may form a source‐sink system in which abyssal populations are regulated by a balance between chronic extinction arising from vulnerabilities to Allee effects and immigration from bathyal sources. An increased significance of source‐sink dynamics with depth may be driven by the exponential decrease in organic carbon flux to the benthos with increasing depth and distance from productive coastal systems. The abyss, which is the largest marine benthic environment, may afford more limited ecological and evolutionary opportunity than the bathyal zone.

New records, species, genera, and a new family of gastropods from hydrothermal vents and hydrocarbon seeps*

More than 100 species of gastropods from vent and seep localities around the world are reviewed, based on literature information and new material.

Novel Forms of Structural Integration between Microbes and a Hydrothermal Vent Gastropod from the Indian Ocean

ABSTRACT Here we describe novel forms of structural integration between endo- and episymbiotic microbes and an unusual new species of snail from hydrothermal vents in the Indian Ocean. The snail houses a dense population of γ-proteobacteria within the cells of its greatly enlarged esophageal gland. This tissue setting differs from that of all other vent mollusks, which harbor sulfur-oxidizing endosymbionts in their gills. The significantly reduced digestive tract, the isotopic signatures of the snail tissues, and the presence of internal bacteria suggest a dependence on chemoautotrophy for nutrition. Most notably, this snail is unique in having a dense coat of mineralized scales covering the sides of its foot, a feature seen in no other living metazoan. The scales are coated with iron sulfides (pyrite and greigite) and heavily colonized by ε- and δ-proteobacteria, likely participating in mineralization of the sclerites. This novel metazoan-microbial collaboration illustrates the great potential of organismal adaptation in chemically and physically challenging deep-sea environments.

Migration, Isolation, and Speciation of Hydrothermal Vent Limpets (Gastropoda; Lepetodrilidae) Across the Blanco Transform Fault

The Sovanco Fracture Zone and Blanco Transform Fault separate the Explorer, Juan de Fuca, and Gorda ridge systems of the northeastern Pacific Ocean. To test whether such offsets in the ridge axis create barriers to along-axis dispersal of the endemic hydrothermal vent animals, we examined the genetic structure of limpet populations previously identified as Lepetodrilus fucensis McLean, 1988 (Gastropoda, Lepetodrilidae). Mitochondrial DNA sequences and patterns of allozyme variation revealed no evidence that the 150-km-long Sovanco Fracture Zone impeded gene flow between the Explorer and Juan de Fuca populations. In contrast, the 450-km-long Blanco Transform Fault separates the limpets into highly divergent northern and southern lineages that we recognize as distinct species. We describe southern populations from the Gorda Ridge (Seacliff) and Escanaba Trough as Lepetodrilus gordensis new species and refer northern populations from the Explorer and Juan de Fuca ridge systems to L. fucensis sensu stricto. The species are similar morphologically, but L. gordensis lacks a sensory neck papilla and has a more tightly coiled teleconch. To assess the degree of isolation between these closely related species, we used the Isolation with Migration method to estimate the time of population splitting, effective sizes of the ancestral and derived populations, and rates of migration across the Blanco Transform Fault.

The Abyssal molluscan fauna of the Norwegian sea and its relation to other faunas

Abstract About 90 000 specimens of mollusks, from depths between 2 500 and 4 000 m in the Norwegian Sea, have been determined. They belonged to 25 species. Five species are described as new, Cuspidaria centobi (Bivalvia), Anekes undulisculpta, Crinolamia dahli, Cylichna lemchei (Gastropoda), and Siphonodentalium laubieri (Scapho-poda). Two of these, Anekes undulisculpta and Crinolamia dahli are type species of the new genera in which they are placed. The other species have been critically examined from a taxonomical point of view, as have the species previously recorded from the Polar Abyssal Basin, but not present in the Norwegian Sea. Some information is given about the feeding, anatomy, and larval development of the species. The origin and development of the fauna are discussed, and it is concluded that at least a part of the fauna is a reminiscence of the old North Pacific fauna that lived in the Polar Basin before this was disconnected from the North Pacific by the formation of Bering Strait. These s...

Planktotrophic development in deep-sea prosobranch snails from the western North Atlantic

Abstract The incidence of planktotrophic development in prosobranch snails increases with depth seaward of the continental shelf in the western North Atlantic. This pattern may result from both stronger selection for dispersal ability to track patchy prey resources and relaxed restrictions on dispersal with increasing depth.

The fauna of hydrothermal vents on the Mohn Ridge (North Atlantic)

Abstract The macrofauna of the newly discovered hydrothermal vent field on the Mohn Ridge at 71°N was investigated. Samples were collected during the cruise BIODEEP 2006 using the ROV ‘Bathysaurus’. A total of 180 species-level taxa were identified. The region contains very few vent-endemic species, but some species of Porifera, Crustacea and Mollusca may be vent-associated. Dense aggregations of motile non-vent species such as Heliometra glacialis and Gorgonocephalus eucnemis surrounded the vent area, but the area in general only held small numbers of sedentary animals. Calcareous sponges comprised an unusually high portion of the sponge species found and they constitute one of the first pioneers among the sessile invertebrates settling on these vents. Possible explanations for the structure of the fauna in the region are discussed.

Rubyspira, New Genus and Two New Species of Bone-Eating Deep-Sea Snails With Ancient Habits

Rubyspira, a new genus of deep-sea snails (Gastropoda: Abyssochrysoidea) with two living species, derives its nutrition from decomposing whalebones. Molecular phylogenetic and morphological evidence places the new genus in an exclusively deep-sea assemblage that includes several close relatives previously known as fossils associated with Cretaceous cold seeps, plesiosaur bones, and Eocene whalebones. The ability to exploit a variety of marine reducing environments may have contributed to the evolutionary longevity of this gastropod lineage.

A hydrothermal seep on the Costa Rica margin: middle ground in a continuum of reducing ecosystems

Upon their initial discovery, hydrothermal vents and methane seeps were considered to be related but distinct ecosystems, with different distributions, geomorphology, temperatures, geochemical properties and mostly different species. However, subsequently discovered vents and seep systems have blurred this distinction. Here, we report on a composite, hydrothermal seep ecosystem at a subducting seamount on the convergent Costa Rica margin that represents an intermediate between vent and seep ecosystems. Diffuse flow of shimmering, warm fluids with high methane concentrations supports a mixture of microbes, animal species, assemblages and trophic pathways with vent and seep affinities. Their coexistence reinforces the continuity of reducing environments and exemplifies a setting conducive to interactive evolution of vent and seep biota.